The long-term goal of this research project is to understand the function of AML1, also known as Runx1, in hematopoiesis. AML1 is a DNA binding transcription factor that is first expressed in hematopoietic stem/progenitor cells (HSPCs) and regulates how they differentiate into mature blood cells. AML1 is often considered a master regulator of hematopoiesis. Analyses of AML1 knockout (KO) mouse models demonstrate that AML1 plays a fundamental role in definitive hematopoiesis. Studies of adult mice with AML1 conditionally knocked out showed these mice have abnormalities in normal blood cell maturation and that their HSPCs were defective in competitive hematopoietic cell repopulation, maintenance and self-renewal. AML1 was originally cloned from human acute myeloid leukemia cells with a translocation involving chromosomes 8 and 21. In addition, patients with mutations in AML1 or with haploinsufficiency of AML1 were linked to the development of myelodysplastic syndrome, myeloproliferative disease, and familial platelet disorder with propensity to develop acute myeloid leukemia. These results have revealed that AML1 serves a vital role in both blood cell differentiation and blood-associated diseases. Here, I propose to continue the characterization of AML1 function by testing the hypothesis that direct targets of AML1 regulate critical functions in HSPCs. We have already performed differential gene expression studies on wild type and AML1-deficient HSPCs, and have conducted genome-wide analysis of AML1 transcription factor occupancy. To further establish the role of AML1 in HSPCs, I will first confirm that AML1 interacts with the loci of its target genes to affect gene expression by using chromatin immunoprecipitation-quantitative PCR studies and promoter-luciferase assays. Second, the expression and/or activity of candidate target genes will be modulated in the background of AML1 deficiency and in vitro analyses including colony formation, colony replating, and long-term culture initiating cell assays will be performed. These assays will reveal how interactions between AML1 and its target genes may alter the functions of HSPC differentiation, self-renewal, and long-term repopulating potential. Finally, the in vivo functions of these target genes in AML1-deficient HSPCs will be tested using transplantation studies and competitive hematopoietic cell repopulation assays. These three Specific Aims will result in the identification and characterization of AML1 direct targets and their functions in AML1 related blood development. Overall, the insights revealed in this study will provide a better understanding of hematopoiesis, leading to improved treatments for blood-related disorders and better methods for bone marrow transplantation.